The purpose of this project is to study immune responses in man and experimental animals to natural and experimental rickettsial infections. Currently, we are study in guinea pigs the long-term prophylactic qualities of epidemic typhus vaccines, both commercial and experimental. Studies of sera from volunteers previously sensitized to Coxiella burnetii and then given 0.2 microgram of Q fever vaccine have involved isolation, purification and characterization of the resulting antibodies by a variety of chemical, physical and immunologic techniques. An investigation of Legionnaires' disease was undertaken because of the unexpected discovery of antigenic relationship between the bacterium of Legionnaires' disease and the 6BC strain of Chlamydia psittaci. The bacterium of Legionnaires' disease was characterized with respect to antigenic relationships with other microorganisms, pathogenicity and limits of infectivity in guinea pigs, behavior in tissue culture systems, antibiotic susceptibility and strain variation.